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Ask SAIL: Topping Off With Solar

I was reading “Seeing the Light” (May 2014) about solar power on a budget, and it got me thinking about a similar problem I’ll be facing soon. This summer I will be moving my boat to a dock where there is no shore power for my battery charger.

Q:Frank Lanza, Kema, TX

I was reading “Seeing the Light” (May 2014) about solar power on a budget, and it got me thinking about a similar problem I’ll be facing soon. This summer I will be moving my boat to a dock where there is no shore power for my battery charger. I have moved it there before for a season and suffice it to say my batteries didn’t survive. I had to replace them. I don’t need anything anywhere near as advanced as the one mentioned in your last issue. All I want to do is put a 2.5- to 5-watt panel on my batteries just to keep them maintained. Would this do the job? And if so would I still need a regulator hooked into them to keep them from overcharging? I have read that on such a low wattage, that it is not necessary.

Nigel Calder Replies

Without knowing the of the battery bank, I can only say that the rule of thumb I use is that a panel output of around 3.6 watts is needed for every 100AH of battery capacity being floated. If the solar panel output is much higher than this, you will also need a regulator. But if not, you can get away without a one. Note that the 3.6 watts presupposes there are no loads whatsoever on the battery. If you have even one or two LEDs, like those that are often present in bilge pumps and other circuits, these are likely to drop the charge rate below an effective float charge.

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